1. Technical Field
The present invention relates to transformerless subscriber's telephone circuits and more particularly to a subscriber's telephone circuit employing both integrated circuit elements and discrete components in an active gain circuit arrangement.
2. Background Art
Since the advent of integrated circuit technology, numerous efforts have been made to reduce telephone design programs by the utilization of two or three integrated circuit devices. The objectives of such design programs have been to achieve low cost, high reliability and an increased number of usable features. The provision of an increased number of features has to a great extent been successful. For example, such designs permit the use of pushbutton dialing when connected to telephone offices which are configured to accept only rotary dial type telephones.
However, it should be noted that other objectives have been only partly achieved. This is for the most part due to very stringent circuit requirements found in telephone ciruitry. A conventional telephone circuit for example, would usually be required to operate from a variable supply voltage, withstand both current and voltage surges, provide analog and digital functions and signal through its own power supply circuit. Consequently, most telephone circuits commercially available to date employ several discrete electronic components to support the incorporated integrated circuit based designs.
While integrated circuit technology has been progressing in the direction required for telephone designs over the last few years, substantial improvements have also been made in the discrete semiconductor technology field. Such improvements include a wider variety of low cost transistors and diodes, as well as lower cost resistors and capacitors.
Prior to the advent of semiconductor technology, conventional subscriber's telephones employed circuitry including a hybrid transformer to perform a two-wire to four-wire conversion. This conversion is an essential function of all two-wire analog telephones. During this same period, voltage dependent resistors (varistors) had been used extensively to support such hybrid designs. Varistors were used to provide automatic ajustment of transmit and receive levels and to compensate for various loop lenghts (longer loops requiring higher transmit levels and higher receive sensitivity). An undersirable consequence of utilization of the varistor/transformer type of circuitry is the presence of a variable AC impedance between the ring and tip (L1 and L2) terminals of the telephone. It is actually much more desirable to have a fixed AC impedance (600 Ohms) telephone connected to a line so as to maximize the return loss which, in turn, reduces voice signal transmission problems within the telephone system.
One of the first portions of subscriber telephone circuitry to be successfully adapted to the utilization of integrated circuit technology is the pulse dialng portion. Such pulse circuitry typically employs pushbutton inputs to generate pulse outputs. Units of this sort usually draw their power from the telephone line. Most designs of this type employ a current regulator device or current limiting device to prevent dial pulse distortion during dialing. Such pulse distortion is caused by supplying too much loop current to the pulse dialer integrated circuit while trying to maintain a minimum operating voltage for satisfactory integrated circuit operation. Such techniques are disclosed in U.S. Pat. No. 4,167,655 as well as in British Pat. No. 1,426,585.
Accordingly, the object of the present invention is to provide a new and improved subscriber's telephone circuit which employs an optimal combination of discrete and integrated circuit components to overcome many of the disadvantages found in prior art subscriber telephone circuits.